9 The Coastal Sandstones - Sapphire Coast

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Typical aggradational environments include lowland alluvial rivers, river deltas andalluvial fans. Aggradational environments often undergo slow subsidence under theweight of the accumulating sediment which balances the increase in land surfaceelevation due to this aggradation.The timing of the deposition of these two sandstone formations is thought to be quitedifferent. There are uncertainties about the age of the Quondolo Formation butstudies suggest it is Paleocene to Early Eocene, about 55–50 million years ago(Mya).Deposition of the Long Beach Formation is thought to have occurred during theOligocene to Early Miocene (23-30 Mya). This is in accord with the 31 million yeardate for the Coila Basalt, a volcanic flow that is sandwiched between two similarsandstone formations in the Tuross Head part of the region.Aggradation of the south coast mid-Tertiary sandstones is thought to be attributed toan increase in sea level rise during the Oligocene to Early Miocene rise resulting inthe development of coastal barriers of transported sediment at higher levels thantoday.The outcrops seen today are a remnant of the original extent of the sandstones. Thediagram below matches up the contours of current outcrops and remnants that couldconservatively indicate the extent of the aggraded sediments which, in the formingbasins, consolidated into sandstone.
The original extent of these sandstones is indicated in red in the images above.When they were laid down in the early to mid-Tertiary, the landscape was covered byrainforest and might have looked like the debris filled valley in the photograph above.Studies have identified four phases of weathering during the Tertiary recognized inthese deposits. Firstly, there is deep kaolinization (formation of soft white clay underhigh rainfall, well preserved in the deeply weathered volcanics (rhyolite) at WhiteRock south of Tathra. This can be seen on the Kangarutha Walking Trail whichextends from near Wallagoot Lake entrance north to Tathra.White Rock, Bournda National ParkThe second phase of weathering was silicification (formation of silica deposits),exemplified in the Quondolo Formation.Whilst the connection is speculative, it is interesting to note that the local depositionand silicification coincided with a major worldwide increase in silica accumulation(following extensive global volcanism), known as the Silica Burp, and a significantglobal warming event (the Paleocene-Eocene Thermal Maximum at about 55 Mya).It is also noted that the region was then in the 60º latitudes but it was warm andhumid, with a mean annual temperature of 16-22ºC and precipitation over 1.5 metresper year. The vegetation comprised richly mixed rainforests, with plant groupsrelated to those that exist today in the wet tropics rainforests of north Queensland.The traveler’s most common reminders of the region’s Gondwanan heritage is thepresence today of the same tree ferns.Tree Fern – Dicksonia antarcticaRough Tree Fern – Cyathea australisWith a touch of botanical irony, the Norfolk Island pines that form an almostcompulsory addition to Australian beach resorts are ancient Gondwanans. TheirAraucarian relatives would have graced the region’s landscapes in the Cretaceousand early Tertiary periods.
Page 1 and 2: A journey through the earth history
Page 3: The Pinnacles, Ben Boyd National Pa
Page 8 and 9: How Tathra stole the Bega RiverThe
Page 10 and 11: One interesting observation one can

9 The Coastal Sandstones - Sapphire Coast

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